The present invention relates to a wireless terminal providing antenna diversity, for example a mobile phone handset.
Wireless terminals, such as mobile phone handsets, typically incorporate either an external antenna, such as a normal mode helix or meander line antenna, or an internal antenna, such as a Planar Inverted-F Antenna (PIFA) or similar.
Such antennas are small (relative to a wavelength) and therefore, owing to the fundamental limits of small antennas, narrowband. However, cellular radio communication systems typically have a fractional bandwidth of 10% or more. To achieve such a bandwidth from a PIFA for example requires a considerable volume, there being a direct relationship between the bandwidth of a patch antenna and its volume, but such a volume is not readily available with the current trends towards small handsets. Hence, because of the limits referred to above, it is not feasible to achieve efficient wideband radiation from small antennas in present-day wireless terminals.
A further problem with known antenna arrangements for wireless terminals is that they are generally unbalanced, and therefore couple strongly to the terminal case. As a result a significant amount of radiation emanates from the terminal itself rather than the antenna. A wireless terminal in which an antenna feed is directly coupled to the terminal case, thereby taking advantage of this situation, is disclosed in our co-pending unpublished United Kingdom patent application 0108899.6 (Applicant""s reference PHGB010056). When fed appropriately, the terminal case acts as an efficient, wideband radiator.
In many situations it is desirable for a wireless terminal to implement antenna diversity, whereby two or more antennas are used together to improve performance over that which can be achieved with a single antenna. In general, antenna diversity results in better reception, power savings and hence longer battery life. However, provision of two or more conventional antennas in a wireless terminal, such as a mobile phone handset, requires a significant extra volume which is undesirable given the present trend to smaller and smaller handsets.
An object of the present invention is to provide a compact wireless terminal having antenna diversity and efficient radiation properties over a wide bandwidth.
According to the present invention there is provided a wireless terminal comprising a ground conductor and a transceiver coupled to a plurality of antenna feeds, wherein each antenna feed is coupled directly to the ground conductor.
Because the ground conductor (typically a handset body) is used as the radiating element, there is minimal extra volume required to implement antenna diversity (simply the volume occupied by a second capacitor or other coupling element). Hence, the present invention provides antenna diversity with a much-reduced volume requirement compared to known arrangements, while also providing a significantly larger bandwidth. Although the use of two feeds to a common radiating element might be expected to result in high correlation between the two antenna patterns, it is shown that in fact low correlation (and hence good diversity performance) is achieved in practice.
The present invention is based upon the recognition, not present in the prior art, that the impedances of an antenna and a wireless handset are similar to those of an asymmetric dipole, which are separable, and on the further recognition that the antenna impedance can be replaced with a non-radiating coupling element.